Jade fits this trope, as she is often the one to use the Talismans or other artifacts in battle, and has used her Guile Hero staus and Genre Savvy to outwit the Demon Sorcerers and Tarakudo.
Uncle does it as well. Being a man long past his time, he's still a Retired Badass, who'd use everything at his disposal in a fight. Like a gold bar lying nearby when the bad guys tried breaking into Fort Knox. Stamping on feet also proves equally effective.
Terry McGinnis in Batman Beyond is quite fond of fighting dirty. In Terry's case, it actually gets noted in The Movie, where the Joker points out (after being kicked in the crotch) that the real Batman would never fight that way. To which Terry's response is that he's not the same Batman, which he then uses to make a couple other points in an epic verbal beatdown. It was even an Ironic Echo because Joker had made it a point to let Terry know he didn't consider Terry the real Batman.
It just led to the Joker introducing HAND GRENADES to their fight. And then Terry uses the Joker's own joybuzzer on him to short out the microchip, so it all balances out.
In one of their encounters in the G.I. Joe CG movies, Snake Eyes defeated Storm Shadow by pulling the pins off the grenades he was wearing across his chest.
He also dispatched a Red Ninja in the comic by tossing a grenade at him; Larry Hama did the scene as a homage to the Indiana Jones Cairo swordsman scene.
the Season 3 episode, "Sokka's Master" has Sokka learn sword fighting from a Master Swordsman. He's be praised by the teacher for his resourcefulness (e.g. taking advantage of his greater agility compared to someone who is older, attacking from higher ground, pulling bamboo shoots so they'll fly back and distract his opponent, throwing sand in his eyes). Unfortunately for him, his teacher is Crazy-Prepared.
Asami Sato. She drives dirty on the race track, forcing the other car to spin out so she can be sure to win. Later, she plays along with her father until he hands her a weapon, at which point she shocks him right in the chest. If there's a fight scene with Asami in it, she'll matter-of-factly use whatever she has on hand to attack the enemy — including her own car.
Eska and Desna rely a lot on sneak attacks to take out their foes and are big fans of stealth.
Bumi, having only recently gained airbending after a lifetime in the navy, is evidently used to fighting dirty. When you've got the drop on your opponent, biting and hair pulling work just as well as bending.
Suyin Beifong has no problem whatsoever with killing her enemies (in extremely brutal ways, to boot), is perfectly willing to employ assassination attempts to end a stalemate, and once even resorted to attempting to convince Korra to go into the Avatar State and murder an entire army in order to ensure her city remained independent and her citizens were kept safe.
In Transformers Generation One, most Decepticons were Large Hams who liked to taunt and goad the good guys. Not Shockwave, however. Being the logical type he is, he doesn't bother with banter when Spike and Bumblebee transport into his control center, he just opens fire with his Disintegrator Ray. (Also, Bumblebee's assumption that they can safely take cover behind his mainframe because he wouldn't want to blast it proves false - he does so without a second thought.)
Rattrap from Transformers: Beast Wars. He's not a coward... he just doesn't like taking risks in battle, and he doesn't want to die.
Starscream from Transformers Prime also qualifies, lacking Megatron's insistence on defeating his enemies personally and single-handedly or Airachnid's need to let her prey run around or suffer before she goes for the kill. When he is handcuffed and at Arcee's mercy when she learns he was the one who killed Cliffjumper, Starscream immediately resorts to Ain't Too Proud to Beg and seems too terrified to even free himself. When Arcee, in contempt for his cowardice, goes to free him herself (and then kill him), Starscream immediately impales her in the side, frees himself, and proceeds to beat the crap out of her until she gets her Heroic Second Wind.
Soundwave also qualifies. Though he isn't as underhanded as Starscream (as far as we know,) he is Genre Savvy and very pragmatic, always looking for the simplest and most effective solution to a problem. He has no problems with sending Laserbeak to attack a weakened Airachnid. When he fights Wheeljack it's his Combat Tentacles against Wheeljack's Dual Wielding swords over a sonic canon. When Soundwave loses the upper-hand, he grabs the cannon and uses it on Wheeljack. But by far his favourite strategy is simply letting the enemy charge at him, then opening a Ground Bridge to someplace far away in their path.
Of the three main characters in The Boondocks, Riley is much more resourceful than his brother Huey and granddad Robert. Sure he lacks his brother's kung fu or his granddad's belt proficiency, but he will use every tool at his disposal to win a fight. Chances are he learned this from Huey, which would essentially make him a more melee type version of his brother.
An early episode of The Simpsons has Bart trying to do this to Nelson, throwing mud in his face before attacking him. It doesn't work.
This stems from advice Homer gave him on fighting, He also mentioned that "going for the family jewels has been a Simpson family trademark for generations."
Sasha of Titan Maximum has basically one attack in her arsenal: the Groin Attack. It's not so effective against the giant monsters the titular robot fights, but it's very effective against the mostly male members of Titan's Engineering Core. Palmer also goes for the groin a lot, but uses other attacks, too. Finally, even Jodi, normally quite the fair fighter, gets in on the action in the season finale when having trouble beating the superhuman assassin Claire.
The Shredder in Teenage Mutant Ninja Turtles (2003) series. Even though he has his villainous ego, in virtually every fight in the series he also brings his underlings with him no matter who he's fighting, though he never brings out a gun, but maybe because he realizes that nobody with guns ever hit anything.
In the 2012 series, Shredder and Splinter are both this way, with the latter even encouraging it in his students. Fitting, as they are portrayed as having the mindset of, y'know, ninja.
In the Season 1 episode of Justice League that introduces Grodd, the Flash deliberately uses this against him. Flash is wearing a special headband to protect against Grodd's brainwashing helmet. He pushes down and plays on Grodd's helmet briefly and then jumps back and takes off his headband suggesting to Grodd that they just have a fair fight. Grodd replies, "You're a bigger fool than I thought" and attempts to use his brainwashing helmet only for it to malfunction and knock him out. Flash had sabotaged it with his super speed when he jumped on Grodd.
Jonny Quest: The Real Adventures The Season 2 episode, "Digital Doublecross", has Jonny and Jesse trapped in a Quest World game that Surd has placed a virus in which creates evil clones of the two of them. At the end of the episode, after defeating the clones, an image of Surd appears to blow them up anyway saying, "You know I never play by the rules" (though this isn't exactly true; Surd fall victim to Bond Villain Stupidity throughout the series.)
In the Season 2 episode, "Thoughtscape", Lorenzo and some of Surd's mooks break into the Quest compound and capture Race and Hadji while Jonny, Jesse, and Benton are trapped in Quest World. While tied up, Race says to Lorenzo, "Too bad you don't have the guts to face me man to man", however, Lorenzo knows better and doesn't let it bruise his ego, instead just firing back an insult of his own.
Many of the villains in Teen Titans are fond of fighting dirty, but they're not necessarily smart about it. They're balanced out by Slade and Red X, both of whom are skilled, intelligent, and very well-versed in ignoring the rules of combat etiquette. Slade is a top contender for the title of the most outright dangerous enemy the Titans have ever fought, and Red X flat-out curbstomped the Titans in his first (real) fight against them; on top of that, they never did defeat or catch him. Robin is usually a combat pragmatist, which makes sense considering who trained him.
Batman: The Brave and the Bold gives us Wong Fei's most important lesson: "When outmatched...cheat." Batman proved an...adept pupil, to say the least. Surprisingly for a children's show a running theme of the series is that cheating is usually the best way to win.
Futurama has a lot of this. You very rarely see anyone try to fight fair, and if you do, it results in them losing the fight, quickly. Everything from surprise attacks to Grievous Harm with a Body, with your still living friend, to orbital strikes and threatening the enemy with a massive suicide bombing that would destroy their planet has been seen. Mind you, this is often Played for Laughs.
Twilight Sparkle of all characters in My Little Pony: Friendship Is Magic is one of these as seen in Magic Duel. What's a pony to do when she can't best an opponent fair and square? Cheat, of course! Bonus points in that she uses deception, stage magic, and sleight of hand (hoof?) to beat the now magically superior Trixie (who at that point was a retired Stage Magician).
In the season three finale, Kallus attempts to distract Grand Admiral Thrawn in a fight by flinging a helmet at him. Thrawn in turn resorts to some of the most realistically dirty fighting in Star Wars, including Groin Attack and smashing Kallus's face into his knee.
The three rules of the Trollhunter in Trollhunters are all about this. Always be afraid, because fear heightens your awareness while arrogance gets you killed. Always finish the fight by killing your foe so they can't seek revenge or cause further damage. Always kick them in the gronk-nuks (basically troll-speak for genitals) cause its an easy target.
In season 5 of Samurai Jack, Jack has lost his magic sword. As a result, he has resorted to using any weapon he can get his hands on, most notably guns. Most notably, when an armed opponent is down for long enough, he will attempt to take their weapons and use it against them. He took Scaramouch's weaponized tuning fork (a blade that causes objects it strikes to resonate until they shatter) upon defeating him, and he took the sword from one of the Daughters of Aku and immediately aimed for the throat, killing her, Jack not realizing she was not a robot. Jack's style, in general, is also more pragmatic than before, making greater use of the environment and never hesitating to use the opponent's own abilities against them. For instance, when Jack learned about Scaramouch's tuning fork's delayed-shattering effects, he let Scaramouch hit one of Jack's daggers so Jack could throw it at Scaramouch's face right before it shatters.
DuckTales (2017): Dewey learns to be this in order to get Quackfaster to stand down in "The Great Dime Chase!", and later to hold his own against Webby in "The Spear of Selene!".